Mechanism of Grain Refinement of Pulse Current Assisted Plasma Arc Welded Al-Mg Alloy

During welding, the vibration effect of applying a pulse current on the molten pool can effectively improve weld formation and refine grains. The effect of pulse current on grain refinement and its mechanism were studied for Al-Mg alloy welds fabricated by conventional plasma welding (PAW), PAW with conventional pulse current, and PAW with composite pulse current. The grain size produced by conventional PAW was 78.2 mu m, whereas the average grain size was reduced from 78.2 mu m to 53.3 mu m with increasing conventional pulse current frequency from 0 Hz to 100 Hz; in addition, the degree of grain refinement increased by about 30%. However, the minimum grain size was 48.2 mu m, and the grain refinement effect can reach nearly 40% by combining low-frequency pulse current with conventional pulse current. The proportion of small grains and high-angle grain boundaries increased significantly after applying the composite pulse current. The additional oscillation effect of the composite pulse current can effectively eliminate coarse grains during the solidification of the weld pool. The main mechanism of grain refinement is dendrite fragmentation, which is discussed through thermodynamics and composition.